The present disclosure relates to the field of DC power supplies. More specifically, the present disclosure relates to a technique for sensing current flowing through an inductor of a switching DC-DC power supply in order to control the output voltage as a function of load current.
Computer systems are information handling systems that can be designed to give independent computing power to one user or a plurality of users. Computer systems may be found in many forms including, for example, mainframes, minicomputers, workstations, servers, personal computers, Internet terminals, notebooks, and embedded systems. Personal computer (PC) systems, such as the International Business Machines (IBM) compatible PC systems, include desktop, floor standing, or portable versions. A typical computer system is a microcomputer that includes a system processor or microprocessor, associated memory and control logic, and a number of peripheral devices that provide input and output for the system. Such peripheral devices often include display monitors, keyboards, mouse-type input devices, floppy and hard disk drives, optical drives, and printers. The number of devices being added to computer systems continues to grow. For example, many computer systems also include network capability, terminal devices, modems, sound devices, voice recognition devices, electronic pen devices, and mass storage devices such as tape drives, CD-ROM drives, or DVDs.
Typically, computer systems are powered by a power supply system that receives and converts alternating current (AC) power to direct current (DC) power that is used to power the computer system components such as the system processor. In one type of AC-DC power supply used to supply current at DC voltages, power is converted from an AC power source, such as 120 V, 60 Hz or 220 V, 50 Hz power, from a wall outlet. This is accomplished by first rectifying the AC voltage of the power source to an unregulated DC voltage. The unregulated DC voltage typically has a ripple waveform component. To xe2x80x9csmoothxe2x80x9d the ripple component, most power supplies incorporate a bulk filter capacitor or bulk reservoir capacitor. Typically, a bulk filter capacitor stores charge during the ripple peaks and releases charge during the low portion of the ripple cycle. In addition, AC-DC power supplies may typically include a DC-DC converter for providing DC power to the computer system within specified tolerances.
Typical switching DC-DC power supplies incorporate a switching circuit, a controller circuit, resistors, and diodes, in combination with a single-stage LC filter. The typical switching regulator power supply uses a fast operating switch, e.g., a transistor, to switch a DC input voltage through to the output at an adjustable duty cycle. Largely by varying the duty cycle, the average DC voltage delivered to the output could be controlled. Such average voltage consisted of rectangular voltage pulses of adjustable width whose average value was the required DC output voltage. One example of a switching DC-DC converter circuit is a Buck regulator or converter. The Buck regulator circuit is described in further detail in a reference book xe2x80x9cSwitching Power Supply Designxe2x80x9d, Abraham I. Pressman, Second Edition, published by McGraw Hill, ISDN 0-07-052236-7. The output of the Buck circuit may be either a step-down voltage or a boosted voltage.
Advances in processor technology have consistently driven down the supply voltages required to operate processors, thereby reducing power consumption. The supply voltage for processors, which is presently in the +1.0 V to +2.5 V range, may soon extend below 1.0 V. The newer processors, such as Intel""s Pentium class of processors, typically specify a profile or load line that defines the relationship between the processor supply voltage and the current drawn by the processor. For example, Application Note AP-587, xe2x80x9cSlot 1 Processor Power Distribution Guidelinesxe2x80x9d, August 1998, Order Number: 243332-002, published by Intel Corporation describes the power requirements. Typically, the load line is substantially linear and has a negative slope. For example, the higher the voltage, the lower the current drawn, and lower the voltage, the higher the current drawn. Efficient generation of voltages in the +1.0 V range can be a challenge especially when the power supply system is required to produce current outputs of 10 amperes or more.
To measure current passing through the inductor included in the switching DC-DC converter, traditional methods and systems have relied on using a sense resistor connected in series with the inductor. The sense resistor, in this configuration, carries a current substantially equal to the load current. When the load is a processor, the current consumed by the processor is thus substantially equal to the current flowing through the inductor and the sense resistor. One example of a DC-DC switching power supply which uses a controller and a current sensing resistor is the Maxim MAX1718 controller from Maxim Integrated Products, Sunnyvale, Calif. However, a sense resistor continually dissipates power during the normal operation of the power supply, resulting in wasted energy that appears as heat in the power supply.
What is needed is a DC-DC switching power supply, enabled to control the output voltage as defined by a predetermined load line, preferably without a separate current sensing resistor to sense the current flowing through the inductor. Eliminating the need for a current sensing resistor also accomplishes an objective of using a minimal number of components.
In accordance with the present disclosure, a method and circuit thereof for sensing current supplied to a load and establishing an output voltage that conforms to a predetermined load line in a DC-DC converter is described.
In one embodiment, a DC-DC converter circuit includes a DC-voltage input node enabled to receive the DC-voltage signal as an input, an output node enabled to provide an output voltage that varies in response to a load current, an inductor coupled between the output node and an intermediate node, a first switch coupled between the DC-voltage input node and the intermediate node, a current sense circuit having an input coupled to the inductor and a current sense output that varies substantially linearly in response to a current flowing through the inductor and a controller circuit that includes a first controller input coupled to the current sense output, a second controller input coupled to the output node, and at least one controller output for providing the control signal to the first switch. The first switch is operable to vary a duty cycle of a voltage applied to the intermediate node in response to the control signal. Specifically, the control signal causes the duty cycle to vary so that the output voltage varies in response to the current flowing through the inductor.
In this embodiment, the current sense circuit includes an amplifier circuit. The amplifier includes a first input coupled to a first end of the inductor and a second input coupled to a second end of the inductor. The amplifier also includes a feedback network coupled between an output of the amplifier and the first input of the amplifier. The transfer function of the current sense circuit is designed to have a zero attributable to inductor and a pole attributable to the feedback network.
In another embodiment, the DC-DC converter may include means coupled to the inductance for providing a control signal that varies substantially linearly with respect to the load current. The converter circuit may also include means for applying to the control signal to the switch so that the duty cycle of the switch, and the magnitude of the DC output voltage, are determined by the control signal.
In one embodiment, a method of sensing current through an inductor of a DC-DC converter enabled to control an output voltage of the DC-DC converter includes measuring the current through the inductor by measuring a voltage signal across the inductor. In this embodiment, the voltage signal is linearly proportional to the current through the inductor. The output voltage of the DC-DC converter is also measured. The duty cycle of the DC-DC converter circuit is adjusted to vary the measured output voltage. The adjustment is made in response to the measured current flowing through the inductor and the measured output voltage. The adjusted output voltage varies in response to the current flowing through the inductor.
In another embodiment, a method of controlling a DC output voltage at an output node of a DC-DC converter having an input node, an output node and an intermediate node includes sensing a current flowing through an inductor that is coupled between the intermediate node and the output node so as to develop a current sense signal. A control signal is developed in response to the current sense signal and in response to the DC output voltage at the output node. The duty cycle of a voltage applied to the intermediate node is varied so that the DC output voltage varies in response to the current flowing through the inductor.
In one embodiment, the method of controlling an output voltage or sensing current flowing through the inductor, described above, may be implemented in a computer system. The computer system includes a processor, a memory coupled to the processor, and a power supply system for providing power to the processor. The power supply system includes a switching DC-DC converter circuit. In this embodiment, the switching DC-DC converter circuit includes a DC-voltage input, a DC-voltage output that varies in response to a load current, a switch, a current sense circuit and a controller circuit.
In this embodiment, the switch includes a switch input, a switch output and a control signal input, wherein the switch input is coupled to the DC-voltage input node and the switch output is coupled to an inductor. The control signal determines duty cycle of a signal applied to the inductor, the inductor being coupled between the switch output and the DC-voltage output. The current sense circuit includes a current sense input coupled to the inductor to detect a current through the inductor and a current sense output that varies substantially linearly with the current sense input. The controller circuit includes a first controller input coupled to the current sense output, a second controller input coupled to the DC-voltage output, and at least one controller output for providing the control signal to the switch. The control signal provided by the controller circuit causes the duty cycle to vary the DC-voltage output, in response to the current flowing through the inductor.